GB2032722A - A transversal filter for digital signals - Google Patents

Abstract

Taps, from the stages of a delay line (T31 to T3(m-1)) through which the digital signals are shifted, supply first address inputs of a fixed word store (ROM) which has second address inputs supplied with a clock pulse signal (C2). The word produced by the fixed word store depends on the combination of signals at the first address inputs and can be converted in a digital-to- analogue converter (D/A) to provide an analogue output signal of the filter which can be used as a base band filter. <IMAGE>

Description

SPECIFICATION A transversal filter for digital signals This invention relates to a transversal filter for digital signals.

One type of such a transversal filter is known from IEEE Transactions on Communications, Jan. 1976, pages 112 to 118. This publication discloses the production of linear and non-linear, synchronous data signals by digital echo modulation in a time-variable transversal filter. A known form of filter is shown in Fig. 1 and includes a delay line (which consists of stages T11, T12 . . . T1 n), a set of multipliers and an adder circuit. In the event that only digital, and in particular binary signals, are to be processed, the delay line will consist of a shift register driven by clock pulses. In accordance with the sampling theorem, the value of the clock frequency is to depend upon the band width within which the spectrum is to be confined at the output of the filter.For practical considerations it is expedient to use a clock frequency which corresponds to a multiple of the element (pulse) speed of the digital signals which are to be processed. Thus for example in the arrangement disclosed in the above mentioned publication four times the element speed is selected for the clock frequency. For clarity double the element speed is selected for the clock frequency in the following description; the delay time of the individual states T1 1, T1 2... T1 n then amounts to half a clock pulse period which is equal to the duration of the individual pulses of the digital signal.

The input ES for a digital signal and the output of each stage are connected to a signal input of a modulator My 1, M12,.. . Ml n which is supplied with the clock signal via terminals a,, a2, . . . a,n. Thus, as in the publication mentioned above, this arrangement allows the production of modulated digital signals, i.e. which are band-limited in a given frequency range.

The arrangement illustrated in Fig. 1 requires a delay line having a relatively large number, n - 1, of stages and n tappings and a correspondingly high number, n, of modulators (digital multipiiers). However, as the input communication pulses occupy only half an element period, only half of these modulators are ever simultaneously in operation. If the limitation that n should be an even number is imposed, a transversal filter as illustrated in Fig. 2 can be produced with a considerably rediced outlay as a number m, equal to only half the number n, of modulators is required.

Now the duration of the communication pulses is arranged to correspond to the clock pulse period which is equal to the element period; within this period each modulator is used as two modulators by being switched between two evaluation factors (co-efficients).

However, the use of modulators in still relatively large numbers leads to an outlay which in many cases is unviable. Furthermore highly accurate digital multipliers are required in view of the degree of accuracy which is demanded.

It is an aim of this invention to provide a transversal filter which dispenses as far as possible with digital multipliers used as modulators.

According to this invention there is provided a transversal filter for digital signals and including a digital delay line connected to an input for the digital signals, a plurality of tap connections from the delay line and connected to respective first address inputs of a fixed word store, and second address inputs of the fixed word store for connection to a clock pulse signal source, the fixed word store being arranged to supply an output signal determined by the combination of signals at said first address inputs.

This transversal filter emits a band-limited digital signal in digital form. In order to produce an analogue form it is expedient to arrange for the output of the fixed word store to be connected to a digital-analogue converter. The digital signals emitted from such a transversal filter have for example the form sin x x i.e. signals of analogue form.

Apart from being used as a modulator, the transversal filter has particular application as a base band filter for digital signals in which the digital signals remain in the original frequency state. Spectral formation of this kind of digital signals is required for example in so-called roll-off production in which the signal is converted in terms of frequency so as to produce Nyquist flanks.

Embodiments of this invention will now be described, by way of example, with reference to the accompanying drawing in which: Figures 1 and 2 are respective block circuit diagrams of known transversal filters; and Figure 3 is a block circuit diagram of a transversal filter embodying this invention.

The known arrangements shown in Figs. 1 and 2 have been described above and will not be described further.

A transversal filter illustrated in Fig. 3 is connected by its input Esto a source of digital signals, the pulse duration of these signals being Ts. The input Es is connected to a signal input of a delay line which consists of stages T31, T32. . . T3 (m- 1), (in the present case m has been selected to be 6). The input Es for the digital signals is also connected to a first address input of a fixed word store ROM whose other address inputs are each respectively connected to the outputs of the stages T3 1 . .T3(m - 1) of the delay line.

The fixed word store ROM is also supplied via a further address input Etwith a rectangular wave form signal C2 whose frequency corresponds to the bit repetition frequency of the digital signals.

For each possible combination of signals at the address inputs of the fixed word store ROM, the fixed word store provides a digital output signal in the form of a code word comprising 8 bits in parallel at eight terminals of a signal output of the fixed word store.

These signal terminals are connected to input terminals of a following digital-analogue converter D/A which, in known manner, converts the digital input signal to produce an output signal of analogue form which can be obtained from the output A of the transversal filter.

The form of this output signal approximates that of a sin x x oscillation, i.e. this output signal is in analogue form. However, the signal content continues to represent the content of the digital input signal. Generally speaking in a timediscrete-control arrangement, in addition to the desired signal spectrum there will also occur generally undesirable side bands which can be suppressed by a following analogue low-pass filter.

CLAIMS (28 Sep 1979) 1. A transversal filter for digital signals and including a digital delay line connected to an input for the digital signals, a plurality of tap connections from the delay line and connected to respective first address inputs of a fixed word store, and second address inputs of the fixed word store for connection to a clock pulse signal source, the fixed word store being arranged to supply an output signal determined by the combination of signals at said first address inputs.

2. A transversal filter for digital signals and including a multi-stage, digital delay line which is connected to an input for the digital signals, a fixed word store having a first address input connected to the input for the digital signals and further first address inputs respectively connected to outputs of the stages of the delay line, and second address inputs connected to at least one source of rectangular signals, the fixed word store being arranged to emit an output signal having a specific spectral shaping in dependence upon the signals present at its address inputs.

3. A transversal filter as claimed in claim 1 or claim 2, wherein the output of the fixed word store is connected to a digital-to-analogue converter.

4. A transversal filter according to any one of the preceding claims and arranged to operate as a base band filter.

5. A transversal filter for digital signals and substantially as described herein with reference to Fig. 3 of the accompanying drawing.

CLAIMS (9 Jan 1980) 1. A transversal filter for digital signals and including a digital delay line connected to an input for the digital signals, a plurality of tap connections from the delay line and connected to respective first address inputs of a read only memory, and second address inputs of the read only memory for connection to a clock pulse signal source, the read only memory being arranged to supply an output signal determined by the combination of signals at said first address inputs.

2. A transversal filter for digital signals and including a multi-stage, digital delay line which is connected to an input for the digital signals, a read only memory having a first address input connected to the input for the digital signals and further address inputs respectively connected to outputs of the stages of the delay line, and second address inputs connected to at least one source of rectangular signals, the read only memory being arranged to emit an output signal having a specific spectral shaping in dependence upon the signals present at its address inputs.

3. A transversal filter as claimed in claim 1 or claim 2, wherein the output of the read only memory is connected to a digital-toanalogue converter.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. each respectively connected to the outputs of the stages T3 1 . .T3(m - 1) of the delay line. The fixed word store ROM is also supplied via a further address input Etwith a rectangular wave form signal C2 whose frequency corresponds to the bit repetition frequency of the digital signals. For each possible combination of signals at the address inputs of the fixed word store ROM, the fixed word store provides a digital output signal in the form of a code word comprising 8 bits in parallel at eight terminals of a signal output of the fixed word store. These signal terminals are connected to input terminals of a following digital-analogue converter D/A which, in known manner, converts the digital input signal to produce an output signal of analogue form which can be obtained from the output A of the transversal filter. The form of this output signal approximates that of a sin x x oscillation, i.e. this output signal is in analogue form. However, the signal content continues to represent the content of the digital input signal. Generally speaking in a timediscrete-control arrangement, in addition to the desired signal spectrum there will also occur generally undesirable side bands which can be suppressed by a following analogue low-pass filter. CLAIMS (28 Sep 1979)

1. A transversal filter for digital signals and including a digital delay line connected to an input for the digital signals, a plurality of tap connections from the delay line and connected to respective first address inputs of a fixed word store, and second address inputs of the fixed word store for connection to a clock pulse signal source, the fixed word store being arranged to supply an output signal determined by the combination of signals at said first address inputs.

2. A transversal filter for digital signals and including a multi-stage, digital delay line which is connected to an input for the digital signals, a fixed word store having a first address input connected to the input for the digital signals and further first address inputs respectively connected to outputs of the stages of the delay line, and second address inputs connected to at least one source of rectangular signals, the fixed word store being arranged to emit an output signal having a specific spectral shaping in dependence upon the signals present at its address inputs.

3. A transversal filter as claimed in claim 1 or claim 2, wherein the output of the read only memory is connected to a digital-toanalogue converter.

3. A transversal filter as claimed in claim 1 or claim 2, wherein the output of the fixed word store is connected to a digital-to-analogue converter.

4. A transversal filter according to any one of the preceding claims and arranged to operate as a base band filter.

5. A transversal filter for digital signals and substantially as described herein with reference to Fig. 3 of the accompanying drawing.

CLAIMS (9 Jan 1980)

1. A transversal filter for digital signals and including a digital delay line connected to an input for the digital signals, a plurality of tap connections from the delay line and connected to respective first address inputs of a read only memory, and second address inputs of the read only memory for connection to a clock pulse signal source, the read only memory being arranged to supply an output signal determined by the combination of signals at said first address inputs.

2. A transversal filter for digital signals and including a multi-stage, digital delay line which is connected to an input for the digital signals, a read only memory having a first address input connected to the input for the digital signals and further address inputs respectively connected to outputs of the stages of the delay line, and second address inputs connected to at least one source of rectangular signals, the read only memory being arranged to emit an output signal having a specific spectral shaping in dependence upon the signals present at its address inputs.